جستجوی مقالات مرتبط با کلیدواژه « نانوسیال » در نشریات گروه « مکانیک »

Heat exchangers are widely used in various industries, including power plants, solar cells, refineries, and automobiles. One of the simplest types of heat exchangers used in the industry is the double-pipe heat exchanger. In the present study, nanofluid flow in a double-pipe heat exchanger, with a square inner pipe and a circular outer pipe (sc), is simulated and investigated using computational fluid dynamics under constant heat flux and within the range of laminar and turbulent flow regimes. To verify the accuracy of the numerical study in both laminar and turbulent flows, the numerical calculation results for water flow with forced convection were compared with reference results. The results showed that with an increase in the Reynolds number, particularly in the turbulent flow regime, the Nusselt number in nanofluid flow increased more than in water flow. For instance, for aluminum oxide nanofluid flow with a Reynolds number of 500, the Nusselt number increases by nearly 5% more than water flow, and for a Reynolds number of 20000, this Nusselt number increase is close to 20% more. To investigate the impact of nanoparticle type on heat transfer and pressure drop, three types of nanoparticles were considered. The results indicated that the use of nanoparticles had a slight effect on the friction coefficient while significantly enhancing heat transfer.

In the present work, free convection heat transfer of nanofluid in a closed square cavity with a hot circular barrier has been investigated. The location of the circular obstacle and the walls are fixed. The governing equations are solved by finite volume method and using simple algorithm. The purpose of this work is to study the effect of Rayleigh number and adding nanoparticles, taking into account the change in the dimensions of the circular barrier in the cavity, as well as taking into account different density approximation models for modeling the heat transfer of the nanofluid in the cavity. The most important results showed that heat transfer along the walls increased with the increase in the dimensions of the circular barrier, the Rayleigh number and the volume fraction of nanoparticles. Also, the most important results showed that with the increase of the Rayleigh number, the average Nusselt number along the walls and the temperature values increase, in addition to the effect of considering different models for approximating the density in modeling of nanofluid free convection heat transfer in cavity by comparing the thermal parameters It showed that Bozinsk's approximation model provides more favorable and accurate results for investigating free convection heat transfer.

Turbulent backward step flow, including air and copper nanoparticles, has been simulated using the Computational Fluid Dynamics (CFD) method by the Eulerian-Lagrangian method. The simulation was done using two- and three-dimensional methods with CFX and FLUENT software. The obtained results were compared with each other and with the experimental results. The two-way coupling discrete phase model (DPM) was used for simulation. The Saffman lift force, pressure gradient, and turbulence effects on nanoparticles are considered. Numerical results obtained with Eulerian-Lagrangian models and single-phase models in steady and transient have been compared with experimental data. The effect of the turbulence model on the trajectory of particles and in terms of different diameters of 10, 20, 30, 50, 70, 100, and 200 micrometers have been investigated. The effects of particle diameter on the trajectory and behavior of particles and the effect of Stokes number on the presence of particles in the vortex created behind the step have been investigated. The results have been presented as various contours and graphs for two- and three- dimensional, steady, and transient states. Particle trajectories are shown as contours for different Stokes numbers and particle diameters. The continuous phase velocity variation across the channel for different distances of step are presented as graphs. Standard, RNG, and Realizable k-e and standard and SST k-w models are considered for the modeling of turbulent flow. The results show that SST k-w is more accurate than the experimental data. Furthermore, simulation was done using CFX software. Variations of velocity profile are compared with experimental and Fluent data. The results show that the Stokes number and the turbulence model have a significant effect on the trajectory of particles. Three-dimensional modeling of the flow increases the accuracy of the results. The maximum error in the single-phase method is equal to 25% and for the Eulerian-Lagrangian method is equal to 19%. Particles with a Stokes number smaller than 1.2 (equivalent to a diameter of 35 micrometers in this study) sense the presence of the vortex and enter the vortex. Among the turbulence models, the lowest error for the sst model is equal to 6.25, and the highest error for the standard Kε model is equal to 18.75.

The purpose of this work is to investigate a different geometry in the parabolic collector in order to increase the thermal efficiency through simultaneous surface and volume absorption in a combined condition. Recently, it has been observed that if the inlet temperature is above 250 °C, Direct absorption collectors can increase efficiency up to 10% Considering that there is a direct relationship between the concentration of nanofluid and the diameter of the transparent tube carrying the fluid with the absorption coefficient, On the other hand, as the concentration of nanofluid increases, fluid sedimentation and system maintenance problems occur. In this research, the amount of nanocarbon used as a suspension of 0.02 gr/l in oil base fluid is considered. According to experimental data, this amount of nano carbon concentration completely absorbs light rays at a depth of 24 (mm) and this absorption coefficient has been used for this specific geometry of the collector. The results show that the rest of the radiation is absorbed on the surface using the central tube, so that the thermal efficiency is up to 6% compared to the usual copper absorption collector, which is about 19 degrees Kelvin.

the shell and tube heat exchanger with the percentage of baffle cutting and the number of different tube passes in four different modes (use of water fluid on the shell side and tube containing supercritical carbon dioxide gas without the presence of turbulator, use of water fluid on the shell side and tube containing supercritical carbon dioxide gas with the presence of turbulator, the use of water-aluminum oxide nanofluid on the side of the shell and tube containing supercritical carbon dioxide gas without the presence of turbulator and also the use of water-aluminum oxide nanofluid on the side of the shell and tube containing supercritical carbon dioxide gas with the presence of an turbulator) are studied using HTRI software. The results show that the highest value of the heat transfer coefficient on the shell side and, as a result, the appropriate thermal efficiency is related to the case where the baffle cut is 30%. On the other hand, with the increase in nanofluid concentration, the pressure drop on the shell side increases from 4.48 to 5.66%. the results show that the use of a microfin turbulator is more suitable than a twisted tape turbulator and the use of a microfin turbulator increases the heat transfer coefficient on the shell side by 5.76 to 12.77% compared to the use of a twisted tape turbulator. Also, it increases the heat transfer coefficient of the pipe side by 62 and 78% on average, respectively, compared to the case without the turbulator.

 The present study investigates the ability to improve the stability and heat transfer performance of water coolant fluid by dispersing aluminum oxide nanoparticles in water. Surfactant-free nanofluid with 4 different volume fractions of 0.05%, 0.5%, 1% and 2% were prepared by two-step method. The stability of the nanofluid was monitored by continuous imaging (visualization) and dynamic light scattering (DLS) techniques. Thermal conductivity, viscosity and density of aluminum oxide-water nanofluid were measured in 4 concentrations and at temperatures of 25, 35 and 45 °C. The results showed that the thermal conductivity and viscosity of aluminum oxide-water nanofluid increases with rising concentration. The viscosity of nanofluids in all concentrations were higher than the base fluid. This is while the increase in viscosity was almost independent of the increase in temperature. The highest percentage increase in the thermal conductivity of nanofluid is equal 49%, which was obtained at a concentration of 2% and a temperature of 45 °C. New correlations with high precision were presented based on the experimental data of thermal conductivity and viscosity of nanofluids. The heat transfer performance and pumping power of nanofluid were analyzed based on several performance criteria. The advantageous results of increasing the heat transfer capability made aluminum oxide-water nanofluid a potential candidate for cooling fluid in practical applications.

In this study, using Fluent version 2021 software, to investigate the effect of volume fraction, particle diameter in the Reynolds number range of 10,000 to 100,000 using finite volume method and Eulerian-Lagrangian approach for two-dimensional and three-dimensional geometry. The next one is done in a 1 meter tube. Also, the application of thermophoresis and Brownian force and its effect on Nusselt number results were investigated. The data related to all three modes of thermophores and Brownian force and in the absence of these two forces up to Reynolds 60 thousand were in good approximation with the experimental results and the effect of the forces and the difference of the respective Nusselt numbers from Reynolds above 60 thousand can be seen. The results show that with the addition of nanoparticles, the thermal performance coefficient is increased, and the size of the diameter of the nanoparticles and the volume fraction have a significant effect on this performance. The results were validated with experimental values and showed good agreement.

Thermal conductivity of MWCNT(40%)-CuO(30%)-TiO2(30%)/Water nanofluid in different volume fractions and temperatures is modeled and analyzed by artificial neural network method. The type of artificial neural network is MLP. 48 experimental data series are used, 70%, 15%, and 15% are used for training, validation, and testing, respectively. The optimal neural structure has two hidden layers, in which there are 4 neurons in the first layer and 5 neurons in the second layer, with the transfer functions of logsig and tansig, respectively. Neural network training is done with Levenberg-Marquardt (ML) algorithm. The values of regression coefficient, R, and mean squared error, MSE, for the optimal structure are obtained as 0.9995753 and 2.8734E-06, respectively. The correlation equation is also presented to predict the thermal conductivity of nanofluid. The comparison between the correlation equation and the artificial neural network shows the superiority of the artificial neural network. MOD values for artificial neural network are also in the range of -3% to +7%.

In the present study, fluid flow and combined natural and force convection heat transfer of a nanofluid in the horizontal concentric / eccentric cylindrical with different uniform wall temperatures is numerically investigated. The force flow is induced by the cold rotating outer cylinder at slow constant angular velocity, with its axis at the center of the annulus. Moreover, in calculating the buoyancy force caused by temperature difference between annulus, used of the Boussinesq approximation. the results are presented for non-dimensional group number (Reynolds and Rayleigh (.An increase in the Rayleigh number causes non-uniformity of the flow and isothermal lines and increases the heat transfer in the walls. The highest heat transfer related to the two-phase state of non-concentric cylinders was obtained. An increase in the Rayleigh number causes non-uniformity of the flow and isothermal lines and increases the heat transfer in the walls. The highest heat transfer related to the two-phase state of non-concentric cylinders was obtained.

In this research, an experimental analysis of the effect of using aluminum oxide nanofluid (Al2O3) in improving the heat transfer of the engine radiator (XU7) with complete cooling system equipment was done. Tests in three pure water conditions; water and ethylene glycol (60:40) and finally with aluminum oxide nanofluid (Al2O3) with volume fractions of 1% and 2% and flow rates of 10, 21, and 32 liters per minute, in two slow and fast cycles. The cooling fan is done. The results showed that increasing the volume fraction of nanoparticles to the base fluid increases the heat transfer coefficient up to a flow rate of 21 liters per minute, and after that this coefficient decreases. By adding 2 percent by volume of nanoparticles to the base fluid at high fan speed, for flow rates of 10, 21, and 32 Lit/min, respectively, we see an approximate increase of 3%, 20%, and 16% in the displacement heat transfer coefficient compared to the base fluid. The pressure drop of the radiator with a volume fraction of 1 and 2% compared to the base fluid at a constant flow rate of 30 liters per minute was calculated as 22% and 40.8%, respectively.

The pH level of nanofluids plays an important role in stability and thermal conductivity. However limited studies have been done in this field. In this research, the effect of pH on the stability and thermal conductivity of ZnO-EG nanofluid at concentrations of 0.05 and 0.75% volumetric fraction and MgO-W at concentrations of 0.05 and 0.5% volumetric fraction were investigated. Experimental measurements of the thermal conductivity were performed by a thermal properties analyzer device at a constant temperature of 25 °C. The results showed that the pH strongly affected the stability of nanofluids so that at the pH of the isoelectric point (IEP), complete aggregation and sedimentation were observed. The thermal conductivity of nanofluids has the lowest value at the pH of the isoelectric point, but as the pH moves away from the isoelectric point, the thermal conductivity  increases. The highest enhancement in the thermal conductivity of ZnO-EG nanofluid was 63%, which was obtained at a volume fraction of 0.75% and pH = 12. However, the highest enhancement in the thermal conductivity of MgO-W nanofluid was 49%, which was obtained at a volume fraction of 0.5% and pH = 12. Finally, using the experimental results and with the help of curve fitting, equations with good quality were presented to predict the effective thermal conductivity of metal oxide nanofluids.

In this study, the magnetic field effect on the hydrodynamics and heat transfer of the laminar flow of a nanofluid in a microchannel with one-sided sudden expansion at a low Reynolds number is investigated. The governing equations including mass, momentum, and energy conservation equations are discretized and solved on a staggered grid using the finite difference method by developing a Fortran code. The simulation results show that at a low Reynolds number, no vortex is formed after the step. In the presence of a magnetic field, the friction coefficient decreases with volume fraction. At a volume fraction of 0.04, this reduction in comparison with water at Hartmann numbers of 20 and 40 is 25 and 18 percent, respectively. By applying the magnetic field, the friction factor increases. For nanofluid with a volume fraction of 0.04, the enhancement of average friction factor in comparison with water at Hartmann numbers of 20 and 40 is 176 and 337 percent, respectively. At a low Reynolds number, the magnetic field effect on the Nusselt number is negligible. The Nusselt number decreases with volume fraction. At a volume fraction of 0.04, a reduction of about 12 percent is observed in the Nusselt number in comparison with water. Examining the performance evaluation criteria shows that applying a magnetic field improves the system's performance. At a Hartmann number of 20 and volume fraction of 0.04, an enhancement of about 11 percent in performance coefficient is observed in comparison with the case that the magnetic field is absent.

Nanofluids have been highly interested by researchers based on their interesting thermophysical properties and application in important branches of engineering such as heat transfer. However, there are contradictions in the laboratory results and the topics presented in several cases such as effective heat conduction, convection heat transfer coefficient and boiling heat transfer rate. In contrast to convective heat transfer, limited efforts have been made in boiling nanofluids. However, to exploit nanofluids as the next generation of coolants, boiling research is essential. The boiling process in magnetic fluids (a class of nanofluids with magnetic nanoparticles, mainly iron), in addition to showing the general characteristics of the boiling of nanofluids, is remarkable due to its ability to be controlled by the application of a magnetic field.

Today, the use of nanofluids in microchannels is widely used for cooling microelectronic components. In this study, the flow and heat transfer of nanofluid in a converging and diverging microchannel has been investigated. The governing equations are solved by finite element method and two-phase mixture model in Comsol Multiphysics software. The results of this simulation were obtained for Reynolds numbers (100-700) and different concentrations of nanoparticles (0-0.02) for diverging and converging microchannels with different slopes (0-0.05). Also, the effect of two different nanofluids water-copper and ethylene-glycol-copper has been considered in the simulations. Nanoparticles diameter is 50 nm and the average height of the microchannels is 50 microns. The results include Nusselt number and performance coefficient for different situations. For water-copper nanofluid with a volume fraction of 1% in a converging microchannel with a slope of 3% and Reynolds 100, it increases by about 1.6 times for a converging microchannel and 1.1 times for a diverging microchannel compared to a flat microchannel. In this case, the performance coefficient for convergent and divergent microchannel is 1.37 and 1.74, respectively. In the same conditions, for ethylene glycol-copper nanofluid, the Nusselt number for converging microchannel becomes 1.22 times compared to flat microchannel and 1.13 times for divergent microchannel. In this case, the performance coefficient for convergent and divergent microchannel is 1.17 and 1.4 respectively.

Parabolic solar collector, use highly reflective materials to collect and concentrate the heat energy from solar radiation. The thermal efficiency of the solar collectors increases by using nanofluid. It is necessary to consider some thermal properties of nanofluid and operating parameters to reach maximum efficiency. This article presents the heat transfer of a nanofluid in the absorber tube of a parabolic collector with non-uniform heat flux on the wall. Two types of nanofluids were utilised and prepared TiO2-Syltherm 800 and Al2O3-water with 1.0%, 2.0%, 3.0%, and 4.0% nanoparticle volume fractions are used as working fluids. Reynolds number is between 10,000 and 80,000. Comparing the Nusselt number of TiO2-Syltherm 800 and Al2O3-water nanofluid demonstrated that the nanofluid has better thermal performance than the base fluid. The highest percentage increase in Nusselt number of TiO2-Syltherm 800 and Al2O3-water compared to their base fluid is 66% and 57%, respectively. The highest increase in Nusselt number is related to TiO2-Siltherm 800 nanofluid. at Reynolds 10000 and concentration of 4%. In this case, the Performance evaluation criterion, PEC, is 2.93. For both nanofluids, PEC increases with increasing concentration. With increasing Reynolds number, PEC decreases for TiO2-Siltherm 800 and increases for Al2O3-water nanofluid.

In this study, the effect of using louvered strip inserts in heat exchangers on flow and transfer characteristics is numerically investigated. The continuity, momentum, and energy equations have been solved using a finite volume method. The wall of the tube is heated with a uniform heat flux boundary condition. This paper uses a louvered strip insert arrangement (forward) with a Reynolds number of 10,000. The effects of louvered strip slant angle of  and pitch of 50 mm were used for Al2O3 nanoparticles with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, mixed in a base fluid (water) is used. The comparison of numerical analysis results with existing equations has shown a good convergence. The numerical results reveal that the Nusselt number has increased with decreasing the nanoparticle diameter. The results indicate a slight change in the skin friction coefficient when nanoparticle diameters of Al2O3 nanofluid are varied. The Nusselt number increases with increasing nanoparticle volume fraction of Al2O3 /water nanofluid, while it is found that pure water has the lowest Nusselt number value. Also, the nanofluid has reduced the wall's temperature more than the base fluids (water), which indicates the advantage of using nanofluids in improving the system's thermal performance.

In this study, AL2O3+H2O nanofluid was used in a laboratory setting to test the thermal performance of the M13NI engine. AL2O3 nanoparticles were employed in this experiment along with base fluids made of water and ethylene glycol. We employed 20 nm nanoparticles with volume fractions of 1 to 2%. The outcomes demonstrated that the AL2O3 nanofluid made in the first 22 days was stable after being added SDBS sulphate. Additionally, the zeta potential, which was calculated to be 37.7 mv, shows the stability of the nanofluid. The heat transmission and pressure drop rose as the volume fraction of nanoparticles increased, while the merit parameter fell (ratio of heat transfer to pump power). At 1150 RPM and 1% volume fraction of nanoparticles in the water-based fluid, it was found that the heat dissipated increased by 7.2 and 13.1% in comparison to the mixture of water+ethylene glycol and pure water, respectively. When the volume proportion of nanoparticles in the base fluid increases, the radiator's outlet temperature decreases, resulting in a greater difference between the inlet and outlet temperatures and a faster rate of heat transfer. By utilizing nanofluid, it is possible to lower the radiator's size and the volume of the cooling system, so reducing the volume of circulating water and the engine's wasted power.

Cavity receivers are the most common design in solar dish concentrators in order to achieve high thermal performance. In the current research, in order to design the solar receiver optimally to increase the output fluid temperature, the variables of diameter and pitch of a specific type of receiver, fluid type and velocity and the flow regime have been investigated. The most suitable dimensions of the receiver and flow Reynolds are according to the parameters that lead to the highest fluid outlet temperature. In this study, a spherical concentrator with a spherical cavity receiver is considered. The results show, the most suitable diameter of the receiver tube and its corresponding pitch is 5 mm and 17 rings, the maximum temperature of the outlet fluid is obtained at Reynolds 200 and slow flow regime. Also, the temperature of the outlet fluid with the working fluid of oil and nanofluid oil with nanoparticles have been compared with each other on different days of the year. Therminol with temperature function properties has been used. Nanoparticles of copper, copper oxide and alumina in volume percentages of 0 to 5 % have been added to the oil and the temperature of the nanofluid outlet has been calculated. The results of this comparison show that by adding nanoparticles to the oil, the temperature of the output fluid will increase. The most suitable nanoparticle is copper with a concentration of 5%, which leads to an increase in the temperature of the output fluid by 118 Kelvin degrees.

In this study, mixed convection inside a square cavity with a movable cap and baffle was simulated numerically using the finite volume method. The under-study cavity was two dimensional and affected by gravity and rotated perpendicular to the plane. Right and left walls of the cavity were adiabatic and the upper wall was warm source at a constant temperature. Lower surface was a movable cap that moved from the center to both sides and was assumed to be a cold source at constant temperature. The baffle was assumed to be at the same temperature as cold wall and had a height equal to two thirds of the side of the cavity. Experimental data was used for the thermal conductivity coefficient of the nanofluid. Simulations were performed at a constant Reynolds number to investigate the effects of three parameters of Richardson number, volume fraction of solid particles and cavity slope angle on isothermal lines, streamlines and mean Nusselt value, which created 36 different states. It was found that increasing of slop angle of cavity with respect to reference surface (0 to 90 deg), increasing Richardson number (0.01 to 100) and increasing the volume fraction (0 to 0.05), increase the mean Nusselt value, where the maximum value of which is equivalent to state , , . Increasing the volume fraction of the nanofluid causes an increment in average Nusselt number. It was also observed that at low Richardson values, cavity slope angle has no effect on the results.

In the present paper, mixed convection of water-copper nanofluid in a double lid-driven square cavity consisting of two insulated rotating cylinders is studied numerically. To solve two phase modeling of the nanofluid, the Buongiorno model has been implemented. The governing equations have been solved utilizing the Finite Element Method. The parameters studied in this paper include the effect of different configurations of the cylinders, the rotation direction of the cylinders, the angular speed of the cylinders (Ω), the distance of the cylinders from the moving walls (xc), and the Reynolds number (Re). According to the presented results, the horizontal positioning of the rotating cylinders is the optimal configuration. The rotation direction of the cylinders has an important effect on the heat transfer rate from the hot surface and according to the results, the optimal direction depends on the distance of the cylinders from the walls. Furthermore, with the increase of Re, the heat transfer rate also enhances, so by increasing this number from 50 to 200, the heat transfer rate augments by about 105%.